This invention deals generally with heat transfer and more particularly with a high performance passive two phase cooling loop that is supplemented by a secondary loop with a mechanical pump for moving liquid to the evaporator and excess liquid back to a reservoir.
Future space and terrestrial systems will depend heavily on thermal management technologies that are capable of high performance in compact configurations that are not presently available and for use in environments that are much harsher than those to which the available technology has been subjected. For example, one proposed pulsed laser communication system has a peak power of 4 kW, and present cooling techniques limit the design to a duty cycle of only 0.025%. Continuing trends of increasing heat generated by such equipment and reduced package size will soon result in the requirement for heat removal capabilities exceeding 100 W/cm2. Furthermore, when used for space vehicles such systems must be very reliable, have low mass, and allow significant flexibility in packaging. Until now, the most suitable available cooling devices have been heat pipes, loop heat pipes, and capillary pumped loops. However, each of those devices has some inherent limitations that make them undesirable for use in the new generation of space and terrestrial systems.
Heat pipes are a relatively mature technology. Aluminum and ammonia heat pipes with axially grooved wicks are the current standard of spacecraft thermal control. Copper and water heat pipes with sintered wicks are commonly used in commercial electronics cooling. The primary advantages of the heat pipe technology are the passive operation and nearly isothermal heat transport. However, the heat transport distance and heat flux capability of a heat pipe is limited by the capillary action in the evaporator wick. Most heat pipes are less than 0.5 meter long and operate against a gravity head of no more than a few inches. Typical grooved wick heat pipes cannot handle heat fluxes above 10 W/cm2. Standard sintered wick heat pipes are capable of removing heat fluxes at up to 75 W/cm2, while heat pipes with specially designed wick structures have been demonstrated to handle heat fluxes as high as 250 W/cm2. However, previously demonstrated high heat flux heat pipes were typically subject to heat sources with areas smaller than 1 cm2. As the heat source area increases, boiling starts inside the wick at the center of the heated area, disrupting the capillary driven liquid flow and eventually causing dryout at the center. This presents a serious hurdle to using heat pipes for applications that require removal of high heat fluxes from large surfaces.
Loop heat pipes and capillary pumped loops are passive, two phase flow heat transfer devices that provide greatly increased heat transport capabilities compared to heat pipes. The use of loop heat pipes and capillary pumped loops in spacecraft thermal control systems has been increasing substantially in the last several years. The main difference between the two devices is the construction and location of the compensation chamber or reservoir. One of the shortcomings of loop heat pipes and capillary pumped loops is the limited heat flux capability. Ammonia loop heat pipes, the most common type, cannot handle heat fluxes above 70 W/cm2. Another shortcoming of the loop heat pipe is the difficulty in accommodating multiple evaporators in one loop. In addition, the evaporator of a loop heat pipe or a capillary pumped loop is limited to a cylindrical configuration with diameters of at least 0.25″ for pressure containment and heat leakage reduction. This limits the use of these technologies in compact systems requiring low profile, planar evaporators.
There is a need for more advanced thermal technologies that not only are capable of acquiring, transporting, and dissipating high heat fluxes but also provide substantial mass reduction, reliability improvement and packaging flexibility.